The extensions we build should behave like a coherent suite of applications. Same elements should be the same across all of them. Same design patterns should be used to solve the same design challenges.
From the design side, we should strive to use the same design elements across our designs, when applicable of course. Ideally, most of our designs should be built out of reusable components, and we should minimise the need for once-of custom designs as much as we can. This, of course, cannot be achieved all of the time as each extension has its own goals and needs, so some parts will have to be custom built for that particular extension.
Designers should be aware of the various commonly used elements, such as input fields, check boxes, buttons, tables, and should check for the correct usage of those across the extensions.
It's not enough that a particular element looks good when implemented within one extension, but rather that all instances of that element are the same across the extensions.